1. Field of the Invention
The present invention relates to a liquid crystal composition suitable for use mainly in an active matrix (AM) element and an AM element comprising the composition.
2. Related Art
A liquid crystal display element is classified, based on an operating mode of a liquid crystal, into phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment and the like. It is classified into passive matrix (PM) and active matrix (AM) based on a driving mode. PM is classified into static, multiplex and the like, and AM is classified into thin filter transistor (TFT), metal insular metal (MIM) and the like. TFT is classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to the production process. A liquid crystal display element is classified, based on a light source, into a refection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both.
These elements comprise a liquid crystal composition having suitable characteristics. To improve the general characteristics of the composition can lead to the production of an AM element having good general characteristics. The general characteristics of the composition are related with those of an AM element as summarized in Table 1. The general characteristics of the composition shall be explained further based on an AM element on the market. The temperature range of a nematic phase is related with the temperature range wherein the element can be used. Desirable upper temperature limit of a nematic phase is 70° C. or more and a desirable lower temperature limit is −20° C. or less. The viscosity of the composition correlates with the response time of the element. A short response time is desirable for displaying a moving image. Accordingly, the composition desirably has low viscosity, and low viscosity at low temperature is more desirable.
TABLE 1General characteristics of a liquid crystalcomposition and an AM elementGeneral Characteristics ofGeneral Characteristics ofNothe Compositionan AM Element1Wide temperature range ofAvailable in widenematic phasetemperature range2Low in viscosity1)Short in response time3Suitable optical anisotropyHigh in contrast ratio4Low in threshold voltageLow in electric powerconsumption and high incontrast ratio5High in specific resistanceHigh in voltage holdingratio and high in contrastratio1)The liquid crystal composition can be injected into a cell in a short time. 
Optical anisotropy of the composition correlates with the contrast ratio of the element. To attain a maximum contrast ratio of a transmission type AM element, a product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the element should be designed to approximately 0.45 μm. Accordingly, the optical anisotropy of the composition is primarily in the range of 0.08 to 0.12. On the other hand, in the case of reflection type and semi-transmission type AM elements, Δn·d is approximately 0.25 μm and the cell gap is 2.0 to 4.0 μm in consideration of the response time and the process technology. Accordingly, the optical anisotropy of the composition is primarily in the range of 0.06 to 0.075. Low threshold voltage of the composition contributes to low electric power consumption and high contrast ratio in the element. Accordingly, low threshold voltage is desirable. High specific resistance of the composition contributes to high voltage holding ratio and high contrast ratio of the element. Accordingly, the composition having high specific resistance in the initial stage is desirable. Further, it is desirable that the composition still has high specific resistance even after a long-term use.
A liquid crystal composition having a small optical anisotropy, low threshold voltage and high specific resistance is especially desirable for a reflection type or semi-transmission type AM element so that it is driven with low electric power consumption and displayed at a high contrast ratio. The conventional liquid crystal compositions are disclosed in the following patent documents; JP-A 10-204016/1998, JP-A 10-204436/1998 and JP-A 288470/2001